Energy-efficient house vs. code house: cost, savings and comfort.

Priced as BuildAtPeace building it: only the extra material and extra labor, at cost, no markup. Labor hours are multiplied by the labor rate. Heating and cooling equipment uses typical subcontractor pricing. 2026 Front Range estimates; replace any number with your own and every result updates.

• Code house: built to the Colorado Model Low Energy and Carbon Code (2024 IECC basis), Climate Zone 5 above 4,000 ft: walls U-0.045, ceiling U-0.024, windows U-0.30, R-10 slab edge, 3.0 ACH50, exhaust ventilation, L-shaped plan, windows spread evenly, gas furnace. Some jurisdictions still enforce the 2021 IECC, and the new code also requires extra efficiency credits that aren't modeled, so a real code house may be slightly better than this.
• Geometry: floor area and stories set the footprint. Shape sets perimeter, roof surface, corners and how hard the house is to seal. The golden section is a 1.618 : 1 rectangle. Slab-on-grade, 9 ft ceilings.
• Building types: wall U-factors are whole-wall values including framing. Perfect Wall, ICF and SIP cut corner and junction losses and make air sealing cheaper; ICF adds some thermal mass. SIP walls come in panel thicknesses.
• Heating: monthly heat balance for Denver (NOAA 1991–2020 temperatures, approximate NREL solar on vertical and 55°-tilted surfaces), split into cold, cloudy days and typical sunny days, with ISO 13790 gain use scaled by thermal mass. Screening model, not a certified HERS or Phius model.
• Orientation: turning the main glass wall east or west moves its winter sun toward east/west values. West-facing glass adds summer afternoon heat; east-facing adds a little less. Glass on that wall is capped at 60% of the wall.
• Cooling: degree-day estimate plus summer sun. Equipment is sized from design heat loss at 1°F outside and from peak cooling load.
• Hydronic solar: flat-plate collectors at about 55° tilt, 42% average useful efficiency after storage and piping losses, covering at most 85% of any month's space heat. Collector output left over also preheats domestic hot water (up to 75% of it), credited at the backup's fuel cost. Collectors, storage and radiant floors are priced at builder cost; radiant floors replace ductwork. The default collector size covers about 70% of the house's heating plus hot water. Cooling by ductless mini-splits.
• Default prices: Xcel natural gas about $1.05/therm all-in (Oct 2026), Colorado residential propane $2.30/gal (EIA), Colorado residential electricity 17.1¢/kWh (EIA, June 2026). Edit them in step 5.
• Energy: bills cover heating, cooling, ventilation fans and the household's general electricity (lights, appliances, plug loads: 2,500 kWh plus 2.2 kWh per sq ft a year, the same in both houses). Water heating and cooking are left out, except the hot water that hydronic solar collectors supply. Natural gas or propane is used by both houses for gas-burning equipment; propane at 91,500 Btu per gallon. Savings rise each year with your energy price increase; nothing is financed, discounted or invested.
• Heat distribution and cooling: forced air uses ductwork; in-floor radiant replaces it and uses a boiler or air-to-water heat pump, with ductless mini-splits or ducted central AC for cooling. Equipment is medium grade and sized from the house's design heat loss (1°F outside) and peak cooling load.
• Solar PV: sized to the share you choose of the house's yearly electricity (household use plus electric heating, cooling and fans), at 1,450 kWh per kW per year for the Front Range, priced at builder cost per watt plus permit, interconnection and panel work. Valued at the retail electricity price under Xcel's full retail net metering; panel output declines 0.5% a year. The 30% federal credit (25D) ended Dec 31, 2025.
• Living comfort index: six weighted parts (even temperatures 25%, drafts 20%, air quality 20%, steady temperatures 15%, humidity 10%, quiet 10%). Surface temperatures are calculated for a 10°F night with 68°F inside; the other parts score airtightness, ventilation type, thermal mass, overheating risk, heat distribution (forced air or in-floor) and wall and window type. Fuel and heat source (gas, propane, heat pump, solar) do not affect it. It is a relative index for comparing designs, not a measured or certified standard.
• Incentives: the federal 45L credit ended June 30, 2026. Xcel's new-home program pays $20k or more for qualifying homes with no gas service.